Digital timers for heating purposes have been previously used such as, for example, in electrical resistance stoves. However, in such devices, digital timers perform no more than would be performed by a plurality of conventional mechanical timers since the response time to change cooking functions is limited by the time necessary to heat up and/or cool down a resistive heating element. In addition, due to the resistive nature of the load, the current from a source is substantially uniform.
In cooking systems where heat is supplied by other than resistive heating elements, such as microwave cookers, reactive components of the power supply can produce conditions where current surges of the input power line will blow fuses and/or damage interlock structures. Conventional methods of eliminating such surges have generally relied on the use of reduced voltage starting to limit the initial surge with a mechanical switch shorting out a voltage reducing resistor.
In addition, in microwave cookers, interlock systems are required whose reliability may be reduced by repeated current surges eventually causing the mechanical interlock switches to stick closed or to fail to make contact. Some attempts to minimize such current surges by phase control of AC power switches such as triacs have produced conditions where such current surges are higher than surges produced under average conditions and, hence, require excessively large fuses and interlock current switches. More specifically, if the load supplied is the transformer of a power supply for energizing a microwave cooker magnetron, the condition of the transformer when power is first supplied thereto is important since there may be residual saturation conditions in the transformer so that if it is driven a full excursion from positive to negative, it will drive into saturation with only half the voltage applied thereby becoming a low impedance during the remainder of the current pulse and drawing a substantial current surge.